The spliceosome is a complex macromolecular machinery that catalyses the excision of non-coding introns from a pre-messenger RNA (pre-mRNA). It is formed from five small nuclear ribonucleoprotein subunits (snRNPs) and numerous non-snRNP splicing factors. However, how the snRNPs are organized within a larger unit to execute the catalytic steps of pre-mRNA splicing is not known. By targeting the splicing factor Cdc5p for tandem affinity purification, we have purified a stable 37S spliceosomal-like particle from the fission yeast, Schizosaccharomyces pombe. This complex contains most known U2 and U5 snRNP proteins, the U2, U5, and U6 snRNAs, and numerous uncharacterized splicing factors. Our project proposes to utilize the facile genetic manipulations available in yeast to enhance our understanding of this particle whose composition suggests that it is similar to the human spliceosome arrested at the second step of splicing. Overall, our goal is to determine the three-dimensional structure of this spliceosomal complex using cryo-electron microscopy and single particle reconstruction techniques to provide a first view of the overall organization of a U2, U5, U6-snRNP-containing splicing complex. We also propose experiments designed to further our understanding of spliceosome assembly in fission yeast and protein-RNA interaction within spliceosomal complexes. These studies constitute a significant step towards achieving a more comprehensive understanding of core spliceosome structure and function, which is necessary if we are to elucidate the mechanisms governing splice site selection and protein diversity that impact growth and development.